Transcript G040412-00 - DCC

Flat-Top Beam Profile
Cavity Prototype
J.Agresti, E.D’Ambrosio, R. DeSalvo, J.M. Mackowsky,
M. Mantovani, A. Remillieux, B. Simoni, P. Willems
LIGO-G040412-00-D
Hanford August 2004
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Motivations for a flat-top beam:
Advanced-Ligo sensitivity
(Sapphire Mirror Substrates)
Dominated by test-masses
thermoelastic and coating
thermal noises.
Can we reduce the influence of
thermal noise on the sensitivity
of the interferometer?
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Thermoelastic Noise:
Mirror surface
Created by stochastic flow of
heat within the test mass
Fluctuating hot spots and
cold spots inside the mirror
Fluctuating bumps
Expansion in the hot spots
and contraction in the cold
spots creating fluctuating
bumps and valleys on the
mirror’s surface
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Interferometer output: proportional
to the test mass average surface
position, sampled according to the
beam’s intensity profile.
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Gaussian beam
Mirror surface
As large as possible
(within diffraction loss
constraint).
The sampling
distribution changes
rapidly following the
beam power profile
Larger-radius, flat-top
beam will better average
over the mirror surface.
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Diffraction prevents the creation of a beam with a rectangular power
profile…but we can build a nearly optimal flat-top beam:
Flat-top beam
Gaussian beam
90%
1
e
•The mirror shapes match the phase front of the beams.
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Comparison between the two beams
( Same diffraction losses )
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Indicative thermal noise suppression trends
Substrate thermoelastic noise
Coating thermal noise
Substrate thermal noise
Exact results require accurate information on material properties (Q-factors)
Expected gain in sensitivity ~ 3
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Goals of our project:
Build a small optical cavity to verify the
behaviour of the flat top beams and gain
experience in their generation and control.
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We will investigate the
modes structure and
characterize the
sensitivity to
perturbations when non
Gaussian beams are
supported inside the cavity.
Misalignment produces
coupling between modes
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Design of the test cavity : Rigid cavity suspended
under vacuum
Thermal shield
Flat folding
mirror
Spacer plate
INVAR rod
Vacuum pipe
Flat input
mirror
MH mirror
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folded cavity length
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Optical and mechanical design:
•
Injection Gaussian beam designed to optimally couple to the cavity.
• Required finesse F = 100 to suppress Gaussian remnants in the cavity.
Length stability: ~ 5 nm
• INVAR rods (low thermal expansion coefficient).
• Stabilized temperature.
• Vacuum eliminates atmospheric fluctuations of optical length.
• Ground vibrations can excite resonance in our interferometer structure:
suspension from wires and Geometrical-Anti-Spring blades.
Mirror’s size constrained by beam shape and diffraction losses
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Our Mexican Hat
mirror:
Diameter set by diffraction
losses and technical
difficulties…
Diffraction losses of ~ 1ppm
requires mirror’s radius >1 cm.
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LMA’s Technique to build Mexican Hat mirrors
• Rough Shape Deposition:
• Coating the desired Mexican Hat
profile using a pre-shaped mask
• Achievable precision ~60nm Peak
to Valley
• Corrective coating:
• Measurement of the
achieved shape
• Coating thickness
controlled with a precision
<10 nm.
Maximum slope
~ 500nm/mm
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Cavity Suspensions
V~ 0.6 Hz
H ~ 1 Hz
Suspension System:
GAS spring
wires
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Cavity Vacuum & Thermal Shield
Suspension view
Suspension wires
Vacuum pipe
Thermal shield
Spacer plate
INVAR rod
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Suspension at work!
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Schedule for the future
• Complete building the cavity including the optics and the
electronics
• Lock the cavity with spherical mirror (test the apparatus)
• Switch to Mexican-Hat mirror as soon as available
• Characterization of Flat-top beam modes and misalignment effects
Next possible
developments
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Flat topped beam inside a
nearly-concentric cavity:
same power distribution
over the mirrors but less
sensitive to misalignment.
Overcome the technical
limitation on the slope
of the coating…not
impossible.
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